86 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



tically, bacteria sometimes stick together, and a colony- 

 will occasionally consist of more than one kind of organism. 

 In the majority of instances, however, each colony con- 

 sists of a pure culture. 



A method closely related to the one just described for 

 purification of bacterial cultures is that of streaking. A 

 solid medium such as agar is prepared and the bacteria 

 to be separated smeared back and forth over its surface 

 in an effort to separate the cells from each other. "When 

 successfully done the individual species may be separated 

 in colonies and pure cultures secured. 



It is sometimes possible to use as a medium for plating 

 or for streaking some material which will not allow of the 

 growth of any organisms but the particular one desired. 

 Media of this type are termed differential media. For ex- 

 ample, ox bile is sometimes used to eliminate bacteria which 

 are not of intestinal origin. Some kinds of pathogenic 

 organisms may be secured in pure culture by the device 

 of animal inoculation. If, for example, it is desired to se- 

 cure a pure culture of the Mycobacterium tuberculosis, the 

 organism causing tuberculosis in cattle, from a sample of 

 milk, this may be accomplished most readily by inoculat- 

 ing some of the milk containing the organism into a suit- 

 able animal, particularly, perhaps best, into a guinea pig. 

 Within a few weeks the animal may be killed and the or- 

 ganism will be found growing in pure culture in the lymph 

 glands. 



When once colonies consisting of one kind of organism 

 only have been secured, these may be transferred to other 

 tubes of media by means of the platinum loop or the 

 platinum wire. By successive transfers, it is possible to 

 keep most kinds of bacteria growing indefinitely in the 

 laboratory. 



Before any important study of an organism is under- 

 taken it is customary to plate or in some other way deter- 



